Molecular protonation

The transfer of a proton between an acidic and a basic group within the same molecule is often more complex than the process shown in (1). The proton may be transferred along hydrogen-bonded solvent molecules between the acidic and basic groups if these are too remote to permit formation of an intramolecular hydrogen bond. Alternatively, two inter-molecular proton transfers with an external acid or base may be necessary. Tautomerisation of oxygen and nitrogen acids and bases (3) will be described in Section 6. The reactions are usually quite rapid and fast reaction... 

Finally, atomic and molecular proton affinities (PAs) have also been evaluated for various functionals for ammonia, water, acetylene, silane, phosphine, silylene, hydrochloric acid, and molecular hydrogen. For G2 and G3 theories, the mean unsigned error in PAs is 1.1 and... 

Ultrafast inter molecular proton transfer from 10-hydroxy-... 

To evaluate ot theoretically, one must use perturbation theory. Ramsey did so (see Murrell and Harget, pp. 121-123), and found that o is the sum of two terms, a positive term od (called the diamagnetic contribution, since it decreases the applied field), and a negative term op (the paramagnetic contribution). The term op involves the usual perturbation-theory sum over excited states, and therefore is difficult to calculate however, one can use a variation-perturbation approach (Section 1.10) in op calculations. For molecular protons, od exceeds op, and aH is positive. 

Molecular protonic devices acting as proton wires, rectifiers, charge separation components, etc., represent highly significant goals in the development of information handling systems based on ionic species. They deserve active synthetic and physico-chemical studies. 

Both Ti and T2 relaxations of water protons are mainly due to fluctuating dipole-dipole interactions between intra- and inter-molecular protons [62]. The fluctuating magnetic noise from all the magnetic moments in the sample (these moments are collectively tamed the lattice) includes a specific range of frequency components which depends on the rate of molecular motion. The molecular motion is usually represented by the correlation time, xc, i.e., the average lifetime staying in a certain state. A reciprocal of the correlation time corresponds to the relative frequency (or rate) of the molecular motion. The distribution of the motional frequencies is known as the spectral density function. 

Figure 21-15. Attachment of an excess tt electron to a cyclic hydrogen-bonded cluster facilitates inter-molecular proton transfer (a) formic acid dimer, (b) formamide dimer, and (c) formic acid-formamide (Figure 1 of ref. [51]. Reused with permission. Copyright 2005, American Institute of Physics)...

FIGURE 4.6 Energy diagram for concerted bi-molecular proton transfer from hydrogen bromide to water. 

UV absorbers have been shown to undergo photolysis reactions in polymer materials, the loss being quite critical depending upon the nature and form of the material. Spectroscopic methods have been developed to design more effective UV absorbers which function via intra-molecular proton transfer in the excited state. Other effective stabilizers for polymers includes thioureas for PMMA, aryl nitrenes for PP, diphenylnitrone for polyethylene, zinc glycerolate in PVC o and hexazoclanes for cellulosics and nylons. ... 

V. The Reaction of the Proton with a Molecular Proton Detector. 43... 

V. THE REACTION OF THE PROTON WITH A MOLECULAR PROTON DETECTOR... 

The slower reaction, where the proton reacts with a ground-state molecular proton detector, widens the observation range. It can detect charge modulation of a protein, and the state of the water on an interface. The submicrosecond time resolution can discriminate proton exchange between donor-acceptor moieties located on the same structure from reaction with bulk proton. Similarly, it can resolve a complex proton-binding dynamics into initial and postprotonation events and it determines the contribution of each step to the overall measured equilibrium constants. 

Tautomerism is a combination of hydrogen-bonding and slow inter-or intra-molecular proton transfer. Jarrett et al. (1953) reported proton resonance spectra of acetylacetone (2,4-pentanedione) and 3-methyl-2,4-pentanedione. The spectra were found to be a superposition of... 

DuPre, D.B. (2003) The compressed hydrogen bond in a molecular proton cage. Journal of Physical Chemistry A, 197, 10142-10148. 

To test the quality of both the established and the disputed relaxation models in the light of the new angular-dependent results, we performed simulations and tried model fits of the pertinent theories for proton spin relaxation by reorientations of dipolar coupled inter- and intra-molecular proton pairs. Assuming fast magnetization transfer between unlike spin pairs and a superposition of the three independent reorientations (M) known to be important mechanisms for common nematic liquid crystals, namely nematic... 

When a prochiral ( )-enolate is selectively (Si)-facially protonated, the result is the (H)-enantiomer. (Jle)-Facial protonation leads to the (S)-enantiomer. From the (Z)-enolate, the direct opposite is obtained. If it is not possible to control the ( )/(Z)-configuration of the enolate, in order to obtain good selectivity, one needs then an enantiomericaUy pure acid, whose protonation preference is dependent on the enolate configuration, i.e. for example, it transfers a proton (Si)-facially to the ( )-enolate, but (Re)-fadally to the (Z)-enolate. In many successful cases the enantiomericaUy pure acid is bonded to the metal of the enolate therefore, at the same time it acts also as a Lewis base. In addition, at least from a theoretical point ofview, enantioselective inter- and intra-molecular protonations with achiral acids are conceivable, in which another ligand of the enolate complex is enantionmericaUypure. 

It should be noted here that quantum mechanical considerations are not required for calculation of the total proton transport rate. Inter-molecular proton transfer which might have contributions from proton tunnelling, is triggered by a longitudinal acoustical phonon which can be described by classical mechanics. This is in accordance with a small isotope effect for the equivalent conductance of H and The... 

It is possible to examine experimentally both the diffusion in a liquid crystal of its own molecules (self-diffusion) and the diffusion of impurities. The coefficients of self-diffusion are determined by various methods, e.g., quasi-elastic neutron scattering [100], the diffusion of tracer (radioactive) molecules [101], or the spin-echo methods in nuclear magnetic resonance spectroscopy [102]. The last method is probably the most reliable. Essentially, it is as follows initially the spins of the molecular protons are polarized by a marked pulse of a radio-frequency field then the molecules... 

---